The object of the proposed research is to learn the mechanism by which fiber effects the intestinal microbial population of rats. 1) In rats fed low and high fiber diets, the number and kinds of bacteria present throughout the intestine will be determined, as well as the kinds of sugar substrates and the amounts of organic fermentation acids present. 2) Metabolic studies will be done in order to gain an understanding of each organism's energy metabolism. These will include: (a) growth and yield experiments, (b) analysis of kinds of fermentation products made and the rate at which they are made, (c) enzymatic studies to determine pathways of intermediary metabolism, (d) studies of the anaerobic electron transport systems. The means available to an anaerobic organism for obtaining energy play a major role in determining its ability to compete with other organisms and with its host.